gems, corundum, geology.11, Burma.12, Mogok, beryl, emerald, Russia, Malysheva, Ural Mountains, alexandrite, chrysoberyl, pegmatite

It is widely recognized that some gem minerals often occur together in various localities—either in the same host rock or in the same deposit. Gem minerals each require a certain set of physical and chemical conditions for their formation. This edition of the Colored Stones Unearthed column will explore where specific gem minerals are found together and the conditions that produce these distinct geological settings.

gems, corundum, USA.12, Montana.12, Missouri River, French Bar, Eldorado Bar, sapphire, geology.11

Sapphires have been mined in Montana since being discovered at Eldorado Bar, along the Missouri River, in 1865. The most valuable gold deposit in the area was mined near the top of the French Bar complex from 1867 to 1869. On portions of the Virginia terrace and the Center terrace, the French Bar sill, which sparsely contained sapphires in a host rock—identified here as a basaltic trachyandesite, but described elsewhere as a minette—was reported by Kunz in 1891. Other than Yogo Gulch, where the sapphires are in a lamprophyre dike, the French Bar sill is Montana’s only other source of gem-quality in situ sapphire. New ownership of the property in 2020 has resulted in the largest production to date, although it is still sparse. In this study, the authors examined 80 French Bar sill sapphires in matrix; 16 sapphires removed from the matrix, ranging from very small to 17.99 ct; 4 faceted stones; and 5 partly polished crystals. Trace element chemistry and inclusions were consistent with—and generally indistinguishable from—faceted Montana sapphires from the secondary deposits of the Missouri River, Rock Creek, and Dry Cottonwood Creek. This indicates that while the magmatic material (Eocene volcanism) that transported the sapphires from deep within the earth was different in certain cases, such as with French Bar sill, the genesis of the sapphires may be the same or similar. Many of the surface resorption features are similar, but one important distinction exists. Around 25% of Missouri River alluvial sapphires have encrustations of spinel, likely a result of reaction during transport by a mafic magma. Primary French Bar sill sapphires always have an encrustation of biotite (phlogopite) mica, but never spinel. The French Bar sill itself is certainly too small in extent to have supplied all the alluvial sapphires found at the Missouri River.

gems, corundum, Australia.12, zircon, geology.11, New England District

Transparent pale yellow to brown zircon megacrysts occur with gem corundum in alluvial deposits linked to alkali basalts of the Inverell–Glen Innes District, New South Wales, Australia. Previous LA-ICP-MS analyses of zircons from Kings Plains, Mary Anne Gully, and Swan Brook showed chemical homogeneity and yielded two age groups: 216–174 Ma (Late Triassic–Early Jurassic) and 45–37.7 Ma (Eocene). New U-Pb dating of zircon inclusions in gem-quality blue sapphires produced concordant ages of 37 ± 1.7 Ma (Kings Plains) and 35.1 ± 1.6 Ma (Mary Anne Gully), matching the 38–33 Ma K-Ar ages of the Maybole volcanic suite and aligning with the younger zircon population. These results, together with gemmological and chemical evidence, demonstrate a genetic connection between sapphire crystallisation in the deep crust or mantle and subsequent transport to the surface by alkali basaltic magmas.

gems, tanzanite, zoisite, chromophores, spectra

Zoisite and especially its blue-to-violet variety tanzanite are popular gemstones. While the mineral is found in several places around the world, gem quality material mostly comes from a region southeast of Mount Kilimanjaro in Tanzania. Naturally blue to violet material is rather rare but a simple heat treatment can convert much of the off-colored, yellowish or grayish rough, which represents the majority of the crystals unearthed, to violet or blue colors of various tones and saturation. The source of violetish blue color are small amounts of vanadium substituting for aluminum but little is known that zoisite can also be found in other attractive colors, such as yellow, green, pink or even colorless. In this article we investigate zoisites from Merelani, Tanzania and correlate their colors with the presence and concentration of trace elements, as well as the changes in color after heat treatment. Vanadium in blue, chromium in green, manganese in pink to red and an iron titanium intervalence charge transfer complex in yellow zoisites have been found to be the most important chromophores but also rare earth elements such as neodymium seem to play a role in coloration when present in larger amounts, e.g. in orange stones.

gems, advanced testing, cathodoluminescence, CL

Recently, the author examined a faceted blue oval sapphire. Standard gemological properties and chemical analysis indicated a geographic origin of Kashmir. Microscopic examination revealed internal graining, fine particle banding, and fingerprint inclusions. Interestingly, elongated zircon inclusions were also observed, displaying attractive birefringent interference colors under cross-polarized filters caused by the splitting of light in the doubly refractive crystals. Kashmir sapphire is one of the rarest and most valuable gemstones in the world, renowned for its beauty and quality, and this stone was no exception. [RWH: With brief notes like this, one should not place too much confidence in the origin determination]

gems, advanced testing, cathodoluminescence, CL

A sixteenth-century pendant on display at the Museum of the National Library of France in Paris has been linked to Catherine de’ Medici, Queen of France from 1547 to 1559, and François Dujardin, goldsmith to King Charles IX beginning in 1570. The pendant is adorned with two emeralds that one historian claimed were Colombian without documentation or scientific proof. This study aims to confirm the nature of these emeralds and determine their geographic origin. The microscopic, spectroscopic, and chemical data collected in this study indicate that these two emeralds are from Pakistan. The presence of emeralds from Pakistan in this sixteenth-century European pendant provides strong evidence of the existence of trade routes between Asia and Europe during this period of time, or even before, and suggests such emeralds might be present in other royal jewels.

gems, jade, imitation jade, glass, testing, jade testing, Central Asia, archeogemology,

This paper undertakes a multi-dimensional exploration of the glass imitation jade culture under the context of the ancient Silk Road, tracing the historical trajectory of glass manufacturing technology from West Asia to China, especially after the Han Dynasty, when the flourishing trade along the Silk Road facilitated the integration of foreign glass craftsmanship with China’s indigenous imitation jade tradition. This integration prompted Chinese glass artifacts to not only incorporate foreign techniques but also emphasize the texture and cultural symbolism of jade. Through systematic archaeological excavations and historical text analysis, this study delves into the unique phenomenon of glass imitation jade on the ancient Silk Road in China, providing detailed exposition on the formation, characteristics, and profound impact of this cultural practice. Imitation jade culture extends beyond physical forms, permeating material selection, technological innovation, and cultural symbol inheritance. As a gem of ancient Chinese craft art, the glass imitation jade culture offers valuable insights into understanding the aesthetic features of ancient cultures, cultural exchange patterns, and the evolution of craft technologies. This paper innovatively proposes that ancient glass-making practices imitation jade can serve as a new means to actively protect natural jade resources in modern times, offering a fresh perspective for the exploration and study of the Silk Road cultural heritage.

gems, jade, fei cui, pyroxene jade, jadeite, Myanmar, Burma, Guatemala, Italy

Twenty-seven samples of high-quality green jadeite–omphacite jade from Myanmar, Guatemala, and Italy were characterized to develop a methodology for geographic origin determination and to establish a workflow suitable for commercial gemmological laboratories. The specimens were analyzed using UV-Vis-NIR, FTIR, and Raman spectroscopy, as well as EDXRF and LA-ICP-MS chemical analysis. Statistical processing was applied to compare features and identify discriminant factors. This integrated approach yielded promising and practical results for distinguishing jadeite–omphacite jade from these localities. The most useful elements measured by EDXRF were Ti, Cr, Fe, and Ni, while those identified by LA-ICP-MS were Sc, V, Zn, and Au. For origin determination, LA-ICP-MS provided excellent differentiation power (100% separation rate), and the other four methods individually demonstrated varying separation power (≥67%). The combination of spectroscopic methods with EDXRF chemical analysis effectively compensates for the limitations of each individual technique.

gems, corundum, ruby, Mozambique.12, Madagascar.12, inclusions.5, zircon, primary rutile

In August 2022, a team of GIA gemologists visited the ruby mines near Montepuez in the Cabo Delgado province of northern Mozambique (Fall 2022 GNI, pp. 383–386). This included a visit to the deposits farther west in the region, near the village of Caraia. This mining area is north of the N14 road, while most of the previously known productive ruby deposits are southeast of this highway. Ruby mining near Caraia has been ongoing for more than a decade but was mostly limited to artisanal mining activity until 2017. In the last few years, the ruby deposits have been developed on a larger scale by Fura Gems. The first official sale of this material took place in late 2021, and regular auctions have been organized since then (Fall 2021 GNI, pp. 276–277). During GIA’s visit in August 2022, gemologists collected a large suite of rough rubies directly from the wash plant and sort house at the mine site. The samples ranged from 0.1 to 1.2 g and were transparent and fracture-free. The vast majority contained some crystal inclusions, and an extensive inclusion study of this material matched well with the known data from Mozambican ruby, with one exception. The most common inclusions are bands of fine silk, short needles, and particles, including reflective platelets. Mozambican rubies typically show three distinctive types of solid crystals: transparent greenish amphibole crystals with a stubby to elongated shape; pseudohexagonal frosty mica crystals with small expansion fractures; and black opaque sulfide crystals with a metallic luster. All of these features were prevalent in the rubies mined near Caraia. However, one observation stood out: in 8 of the 85 studied samples, zircon crystal inclusions were found. While zircon crystals are also transparent, their relief is higher than that of the more common amphibole crystals. They also tend to be smaller and are rarely elongated in rubies. The most efficient way to identify them is by viewing the stones between crossed polarizers, which highlights the obvious stress halos around the slightly radioactive zircon crystals (figure 1). In all of GIA’s samples, the identification was also confirmed by confocal Raman spectroscopy. To our knowledge, zircon crystals have not been previously documented in rubies from Mozambique. They are, however, a hallmark for rubies from Madagascar (A.C. Palke et al., “Geographic origin determination of ruby,” Winter 2019 G&G, pp. 580–613). The presence of zircon crystal inclusions is often a decisive discriminator in the origin determination of rubies. The inclusion scenes of high-quality Mozambican and Malagasy rubies can look very similar, but only the latter were known to have abundant zircon inclusions. This is no longer true now that zircon inclusions have also been identified in rubies from Mozambique.

gems, history of gemology, ancient mining, Egypt, emerald, Africa, beryl, Egyptian emerald

This volume offers a detailed study of Ptolemy of Alexandria’s Geographical Guide, whose eight books contain a wealth of geographical information unavailable elsewhere and represent the culmination of the Greco-Roman discipline of geography. Written near the middle of the second century CE, the Geographical Guide is the most anomalous of the surviving works of ancient geographical scholarship but offers a vivid record of the expansion of geographical knowledge in antiquity. Roller examines this peculiar text, which offers unique data about explorations in the far reaches of the inhabited world, from Thoule and Hibernia in the northwest to Kattigara in the southeast, and from Serike in northeastern Asia southwest into central Africa. He positions the Guide within the tradition of ancient geography and gives close attention to the reason why Ptolemy wrote the guide and how it contributes to the genre of geographical scholarship. There is also an emphasis on the topographic and ethnic material within the Guide that is new or unique, especially explorations in sub-Saharan Africa and knowledge of the world beyond India. Because the Guide was written over half a century after the previous extant geographical work―the first books of Pliny’s Natural History―the book also assesses how knowledge of geography changed during this period. This work is an essential text for students and scholars of ancient geography, and is also of interest to anyone working on the cultural history of the Roman Empire during this period.